Treatment of chloralkali feeds containing hydrogen peroxide and base

ABSTRACT

A method and apparatus for removing BHP contaminants (alkali hydroxide and H 2 O 2 ) from a recycled aqueous alkali chloride solution stream before the stream is fed to a chloralkali cell so that the contaminants do not impair the operation of a chloralkali cell. Unwanted alkali hydroxide within the recycled alkali chloride brine solution is reacted with chlorine gas and converted into an alkali chloride, which is useful in the operation of the chloralkali cell, and oxygen gas, which is outgassed from the system. Any H 2 O 2  remaining in the recycled stream after elimination of the alkali hydroxide is reacted with chlorine to form HCl and oxygen gas. The HCl raises the pH of the brine solution, after which the pH may be adjusted by the addition of supplemental alkali hydroxide.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] This invention was made under Contract No: DASG50-00-C-0079awarded by the Army. The government has certain rights in thisinvention.

FIELD OF THE INVENTION

[0002] The invention relates to the recycle of basic hydrogen peroxidefor use in a chemical oxygen iodine laser (COIL) system. Moreparticularly, the invention relates to the purification of a potassiumchloride stream recycled from spent COIL fuel.

BACKGROUND OF THE INVENTION

[0003] Basic hydrogen peroxide (BHP) is a principle fuel used in theoperation of chemical oxygen-iodine lasers (COIL). COIL lasers are shortwavelength high-power chemical lasers with wide ranging industrial,technological, and military applications. COIL lasers, which areelectronic transitional lasers, are favored over vibrational orrotational transition lasers because they have one of the best beamqualities of any available laser, which allows for clean cuts and welds,as well as simple splitting and direction. In addition, the COIL laserhas greater scalability than photolytic and solid state devices.

[0004] BHP is generated by the combination of alkali hydroxide,typically potassium hydroxide, with hydrogen peroxide according to theformula:

OH⁻+H₂O₂+M⁺→O₂H⁻+H₂O+M⁺  (I)

[0005] where M represents an alkali metal such as lithium, sodium orpotassium or combination thereof BHP is the dissociated solution of H₂O₂and MOH. In practice, the term BHP typically refers to a solution having4 molar to 8 molar concentration of perhydroxyl ion (O₂H⁻).

[0006] To power the laser, the perhydroxyl ions and alkali ions of theBHP solution are reacted with chlorine gas according to the formula:

Cl₂+2O₂H⁻+2M⁺→H₂O₂+2MCl+O₂(¹Δ)  (II)

[0007] The gaseous product stream of reaction II is used as a fuel feedto a COIL laser. The singlet delta oxygen (O₂(¹Δ)) of the fuel feed iscombined with a source of iodine, where the excited state oxygen causesrapid dissociation of the iodine. The iodine atoms, excited by reactionwith the singlet delta oxygen, release energy in the form of light.Thus, the excited state iodine is the gain medium for the chemicallaser.

[0008] A feed containing singlet delta oxygen is utilized as fuel forthe laser until the readily usable quantities of O₂(¹Δ) have beendepleted. After being used in the lasing process, the remainingby-products of H₂O₂, KCl, KOH, and water are recycled to form fresh BHP.Typically, the spent fuel stream will be at a temperature below thefreezing point of water. The KCl in the spent fuel, which is onlysparingly dissolved below −20° C., is separated from the H₂O₂ and KOHcomponents of the stream. Water may also be removed from the spent BHP.The liquid stream of H₂O₂ and KOH is recycled as a fresh BHP stream andcombined with chlorine to produce additional singlet delta oxygen. Thewater and KCl from the spent fuel are heated such that the water meltsand dissolves the KCl to form a brine solution. The brine solution maybe fed to a chloralkali cell which uses electrolysis to produce KOH, H₂,and Cl₂.

[0009] In practice, the use of recycled KCl solution with a chloralkalicell for use in a COIL system is often less efficient than would beexpected. In theory, the operation of the cell should be similar to thesodium based NaCl to NaOH chloroalkali cell which is common in the artof caustic soda production. However, the chloralkali cells which userecycled KCl feeds have been plagued with low levels of conversion,performance degradation, and production rate deterioration. Until now,the problems unique to chloralkali cells used in COIL recycle systemshave not been explored. What is needed is a method for improving theperformance of chloralkali cells used in COIL systems.

SUMMARY OF THE INVENTION

[0010] It has been found that when solid alkali chloride, typically KCl,and solid H₂O (ice) are recovered from the spent BHP stream of a COILdevice, the stream of recovered alkali chloride and water containsremnants of BHP, i.e. an alkali base, H₂O₂, and dissociation productsthereof, within or on their solid forms. Therefore, if a stream ofrecovered alkali chloride solution is recycled to a chloralkali reactor,the reactor also receives small amounts of BHP components. It has beenexperimentally determined that the BHP components within the recycledKCl stream form unwanted products within the chloralkali cell, such asO₂, HCl, and ClO₃ ⁻.

[0011] For instance, unwanted KOH within the recycled KCl stream reactswith Cl₂ at the anode of the chloralkali cell to form OCl⁻, which isfurther oxidized at the anode to form ClO₃ ⁻. Each such undesiredoxidation which occurs at the anode of the cell decreases the efficiencyof the cell. Formation of such unwanted products reduces the efficiencyand yield of the chloralkali cell, thereby adversely impacting theoverall efficiency of the COIL process.

[0012] The current invention is a method and apparatus for removing BHPcontaminants from a recycled alkali chloride solution stream before thestream is fed to a chloralkali cell so that the contaminants do notenter the chloralkali cell and do not cause the wasteful production ofunwanted byproducts. Via a number of reaction steps, unwanted KOH withinthe recycled brine solution is reacted with chlorine gas and convertedinto KCl, which is useful in the operation of the chloralkali cell, andoxygen gas, which is outgassed from the system. When sufficientquantities of both KOH and H₂O₂ are present within the recycled brinestream, the main reaction of the invention proceeds according toEquation (1):

2KOH(aq)+H₂O₂(aq)+Cl₂(g)→O₂(g)+2KCl(aq)+2H₂O(l)  (1)

[0013] Because twice the amount of KOH reactant is consumed than H₂O₂according to Equation (1), the brine solution will contain residual H₂O₂after elimination of the KOH components. Any H₂O₂ remaining in therecycled KCl stream after elimination of the KOH is reacted withchlorine to form HCl and oxygen gas according to Equation (2).

H₂O₂(aq)+Cl₂(g)→O₂(g)+2HCl(aq)  (2)

[0014] The HCl lowers the pH of the brine solution, after which the pHmay be adjusted by the addition of supplemental KOH according to theacid base Equation (3).

HCl(aq)+KOH(aq)→KCl(aq)+H₂O(l)  (3)

[0015] The result of the invented method is a recycled brine solutionstream wherein substantially all KOH and H₂O₂ contaminants are removedprior to the stream being fed to the chloralkali cell of a BHPproduction system.

[0016] The invented apparatus is a reactor which uses components of theCOIL process, namely KOH and Cl₂, to pretreat a recycled brine anolytestream before the recharged brine anolyte is supplied to the chloralkalicell, according to the above described process.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Having thus described the invention in general terms, referencewill now be made to the accompanying drawings, which are not necessarilydrawn to scale, and wherein:

[0018]FIG. 1 is a schematic representation of an embodiment of theinvented apparatus, and

[0019]FIG. 2 is a schematic representation of an embodiment of theinvention in the context of a COIL fuel regeneration process.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichpreferred embodiments of the invention are shown. This invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. Like numbers refer to like elements throughout.

[0021] Referring to FIG. 1, the invented reactor 10 is configured toproduce a high-purity, saturated alkali chloride brine solution for useas an anolyte in a chloralkali cell. A stream 12 of low concentrationbrine solution is fed to the reactor 10. The low concentration brinesolution is a depleted anolyte solution, which is removed from achloralkali cell. The components of the depleted anolyte arepredominantly water and a low concentration (5% to 100% depletion,depending upon the particular process) of alkali chloride in solution.The stream 12 may include trace contaminants generated during operationof the chloralkali cell as well as dissolved chlorine produced in theanode chamber of the chlor-alkali cell.

[0022] This invention is broadly applicable to the treatment of alkalichlorides and alkali hydroxides. For ease of description and by way ofexample, potassium chloride and potassium hydroxide are used throughoutthe specification as exemplary salts and bases, and it is understoodthat descriptions with respect to potassium chloride and potassiumhydroxide are generally applicable to alklali chlorides and alkalihydroxides.

[0023] The stream 12 of depleted anolyte is mixed with a stream 16 ofmixed KCl and water which has been recycled from the spent fuel streamof a COIL laser. The recycled KCl stream 16 is typically an aqueous KClsolution that is less than saturated in KCl. The mixture of the depletedanolyte stream 12 with the recycled KCl stream 16 results in a rechargedanolyte solution 24, which is a saturated or nearly saturated aqueoussolution of KCl. It is at least one goal of this process to obtain ahigh-purity, concentrated solution of KCl and water, so any excess wateror KCl present after mixing the two streams 12,16 is either discardedor, preferably, stored for later use in the COIL process.

[0024] The combined stream 24 of recharged anolyte is fed to the reactor10 for further processing. The streams of depleted anolyte 12 and KClsolution 16 are preferably combined and mixed prior to introduction intothe reactor 10. Alternatively, the streams 12, 16 may be separatelyintroduced into the reactor 10 since mixing of the streams will occurwithin the reactor 10.

[0025] Through experimentation, it has been found that residual amountsof unwanted KOH and H₂O₂ are present within a typical stream 16 ofrecycled KCl solution taken from the spent fuel of a COIL laser. Thoughlarge concentrations of KOH and H₂O₂ are desired at a later stage in theoverall BHP recycle process, the presence of unwanted KOH and H₂O₂within an anolyte feed to a chloralkali cell is problematic and leads toa variety of unwanted side products and a decrease in the efficiency ofconversion of KCl to KOH within the cell. In a typical COIL fuel recycleprocess, the amounts of residual KOH and residual H₂O₂ present in theKCl solution 16 occur in a ratio of from 1:1to 1:1.5 (KOH: H₂O₂).

[0026] According to the present invention, the undesired KOH and H₂O₂are reacted out of the recharged anolyte solution within the reactor 10by introducing a stream 20 of chlorine into the reactor 10. The stream20 of chlorine is preferably taken from a portion of the gaseouschlorine output of a chloralkali cell, which is generated from theoxidation of the KCl anolyte within the cell.

[0027] The chlorine reacts with the KOH and H₂O₂ impurities within therecharged KCl anolyte solution 24 according to the Equation (1):

2KOH(aq)+H₂O₂(aq)+Cl₂(g)→O₂(g)+2KCl(aq)+2H₂O(l)  (1)

[0028] The chlorine eliminates the KOH and the H₂O₂ at a 2:1molar ratio(KOH: H₂O₂) for every mole of added Cl₂. The KCl and water productsremain in the refreshed anolyte solution. The oxygen is evolved as a gasfrom the reactor 10. The reaction within the reactor 10 proceedsaccording to equation (1) until the KOH within the brine issubstantially eliminated.

[0029] After elimination of the KOH, the added Cl₂ directly decomposesthe H₂O₂ of the brine solution according to the equation:

H₂O₂(aq)+Cl₂(g)→O₂(g)+2HCl(aq)  (2)

[0030] The chlorine eliminates the H₂O₂ and forms oxygen andhydrochloric acid. The oxygen is evolved from the reactor 10 as a gasvia stream 22, while the HCl remains in the anolyte solution. Additionof the HCl product to the anolyte solution tends to make the anolyteoverly acidic for optimum use within a chloralkali cell. For properoperation of the chloralkali cell, the pH of the anolyte solution ispreferably between about 3 and about 5, and more preferably about 3. Ifthe brine solution anolyte is overly acidic, the hydrogen ions of thesolution compete with the potassium ions in transferring across thechloralkali cell membrane and neutralizing the OH⁻ anions formed in thecathode of the cell.

[0031] To compensate for the acidity gained from the HCl generated as aresult of reaction (2), KOH supplied via a stream 18 of KOH from achloralkali reactor to the reactor 10 is used to neutralize the acidproduced in reaction (2) according to the following equation:

HCl (aq)+KOH(aq)→KCl(aq)+H₂O(l)  (3)

[0032] The KCl and the water produced remain in the anolyte solution. Bythe selective addition of KOH, the pH of the anolyte is maintainedwithin the preferred range of about 3 to about 5.

[0033] At first glance, it may seem contrary to the object of theinvention to introduce KOH into a process which seeks to eliminate KOHfrom a KCl solution. However, the unwanted KOH within the contaminatedstream 16 of recycled KCl is present in a ratio of about 1:1 to about1.5:1 with the unwanted H₂O₂ (KOH: H₂O₂). As shown in equation (1), theaddition of chlorine causes the KOH to be consumed at approximatelytwice the rate of the H₂O₂. Therefore, the additional KOH, as shown inequation (3), must be added to the solution in order to eliminatecompletely the H₂O₂ and to maintain a proper pH within the brinesolution. The KOH addition is controlled so as to produce a pH in theoptimal range, typically between 3 and 5.

[0034] Addition of the supplemental KOH via stream 18 also serves todrive the reaction of equation (1) directly by providing adequate KOHreactant, which causes the added chlorine to react with the H₂O₂ andform the products of equation (1). Utilizing reaction (1) to the fullestextent possible leaves less residual H₂O₂ for the reaction of equation(2).

[0035] It is preferred that the KOH from the KOH stream 18 be reactedwith the recharged anolyte from the recharged anolyte stream 24 in aco-current arrangement. When reacted in a co-current arrangement, theKOH is more likely to take part in the preferred reaction of equation(1) wherein the additional KOH from the chloralkali cell supplements theKOH from the contaminated anolyte stream 24. The small amount of KOHsupplied by the contaminated anolyte stream 24 would otherwise beconsumed and large amounts of unreacted H₂O₂ would remain for reactionaccording to the less desirable reaction of equation (2).

[0036] The supply of chlorine from the chlorine stream 20 is preferablyintroduced to the reactor 10 in a counter-current arrangement withrespect to the recharged anolyte stream 24 and the KOH from thesupplemental KOH stream 18. The countercurrent flow of Cl₂ ensures thatany unreacted H₂O₂ will react with the chlorine according to equation(2) prior to leaving the reactor in the treated anolyte chloralkali cellfeed 14. This arrangement further ensures that the chlorine stream 20 isadsorbed in the greatest extent possible in the combined rechargedanolyte stream 24 and supplemental KOH stream 18.

[0037] Still referring to FIG. 1, in one embodiment of the invention,the reactor 10 is a packed column reactor. The stream 18 of KOH from thechloralkali cell is introduced into the top region of the reactor. Thestream 24 of the contaminated anolyte solution is also introduced intothe top region of the reactor 10. Alternatively, the stream 16 ofrecycled KCl solution and the stream 12 of depleted anolyte from thechloralkali cell are introduced separately into the top region of thereactor. The stream 20 of chlorine is introduced into the bottom regionof the reactor 10 so that the gaseous chlorine flows upward through thepacked column and counter-current to the downward flowing KOH and brinesolutions. The gaseous oxygen generated by reactions within the packedcolumn is allowed to escape through a vent 22. Treated anolyte solution,which is saturated with KCl, contains no appreciable amount of H₂O₂, andhas a pH between about 3 and about 5, is discharged from the reactor viastream 14. Other types of acceptable reactors 10 include a stirredreactor and a falling film reactor.

[0038] The above described method of treatment has been described interms of treating a recharged anolyte solution stream 24 which is thecombination of a depleted anolyte stream 12 and a spent fuel stream 16from a COIL laser. It is to be understood that the contaminants beingremoved by the system reside primarily in the COIL recycle stream 16. Itis within the scope of this invention that the COIL recycle stream 16may alternatively be treated for removal of KOH and H₂O₂ prior tocombination with the depleted anolyte stream 12. In this alternativeembodiment, the COIL recycle stream 16 may first be treated and thencombined with the depleted anolyte stream 12, wherein the depletedanolyte stream 12 does not flow through the reactor, to make a stream ofrecharged anolyte which is substantially free bf KOH and H₂O₂contaminants.

[0039] Referring to FIG. 2, an embodiment of the invented reactor isshown in the context of an Electrochemical (EC) COIL, closed-cycle fuelregeneration process. A COIL laser apparatus 30 is operated, using afuel of BHP 56 and Cl₂ 39. The by-product liquid stream 32 of the COILlaser apparatus is a stream of spent or diminished BHP containing KCl,KOH, H₂O₂, and water. The by-product stream 32 feeds the BHP storagetank 37. Stream 35 carries the diminished BHP to the separationapparatus 34 to precipitate and remove salt and water. The separationapparatus 34 separates the components of stream 35 into two streams, onebeing a stream 16 of aqueous KCl which contains residual amounts of KOHand H₂O₂, and the other aqueous stream 36 which consists mostly of KOHand H₂O₂ and is more concentrated than stream 35. An exemplaryseparation apparatus 34 is a scrapped surface heat exchanger and acentrifuge.

[0040] The stream 16 of recycled KCl solution leaving the separatingapparatus 34 is used as the feed stream 16 of the invented anolytetreatment reactor 10. The stream 12 of depleted anolyte, stream 18 offresh KOH solution, and gas stream 20 of chlorine are each supplied in acontrolled and measured manner to the reactor 10 from a chloralkai cell46. The components of the various input streams 12, 16, 18, 20 interactand react as specified in the description of the reactor 10, above. Apurified, saturated KCl solution anolyte stream 14 from the reactor 10with a pH between 3 and 5 is supplied as treated anolyte to thechioralkali cell 46.

[0041] To complete the COIL fuel recycle process, the majority of thefresh KOH stream 48 generated by operation of the chloralkali cell 46 isfed to a peroxide generator 50. The peroxide generator 50 uses the KOHfeed and may use additional O₂ and H₂O 42 inputs, depending on thechosen mechanism of peroxide generation, to generate H₂O₂ in aqueous andalkaline solution. The generated alkaline solution of H₂O₂ and thedissociation products thereof are then fed as regenerated BHP 52 to theBHP Storage tank 37. As shown, the majority of the chlorine gasgenerated by the chloralkali cell 46 is fed to the chlorine storage tank38 via a chlorine feed stream 54. The recycled BHP 56 and Cl₂ 39 gas areready for use in the COIL laser 30.

[0042] The COIL 30 consumes two moles of KOH for every mole of consumedH₂O₂. The EC COIL regeneration process converts the by-product KCl 16back into KOH and H₂O₂ 52 in the same 2:1 ratio, thereby complementingthe un-reacted-BHP 36 to reconstitute the 1:1 KOH: H₂O₂ BHP solution 56.

[0043] As a result of the invented process, a recycled KCl solution,containing H₂O₂ and KOH contaminants, is treated prior to beingintroduced to the chloralkali cell of a COIL fuel recycling system. Thisresults in a more efficient operation of the chloralkali cell andsubsequent increased efficiency in the overall BHP recycling system.

[0044] Many modifications and other embodiments of the invention willcome to mind to one skilled in the art to which this invention pertainshaving the benefit of the teachings presented in the foregoingdescriptions and the associated drawings. Therefore, it is to beunderstood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

What is claimed is:
 1. A method for removing hydrogen peroxide andalkali hydroxide contaminants from a contaminated aqueous alkalichloride stream comprising reacting the hydrogen peroxide and alkalihydroxide contaminants of the contaminated stream with chlorine gas andsupplemental alkali hydroxide.
 2. The method of claim 1, wherein thecontaminants in the contaminated stream are contacted with saidsupplemental alkali hydroxide in a co-current manner, and with saidchlorine gas in a counter-current manner.
 3. The method of claim 1,wherein the contaminated stream comprises an alkali chloride selectedfrom lithium chloride, sodium chloride, and potassium chloride.
 4. Themethod of claim 2, wherein the contaminated stream is a spent alkalichloride recycle stream from a COIL apparatus.
 5. The method of claim 2,wherein the contaminated stream is formed by the combination of a spentalkali chloride recycle stream from a COIL apparatus and a depletedanolyte stream from a chloralkali cell.
 6. The method of claim 2,wherein the supplemental alkali hydroxide and chlorine gas areintroduced at a molar ratio of between 1:2 and 1:4 (alkali hydroxide:Cl₂).
 7. The method of claim 1, wherein the reactions take place in apacked column, and wherein the contaminated stream and supplementalalkali hydroxide streams are introduced to an upper region of thecolumn, the chlorine stream is introduced to a lower region of thecolumn, and a chloralkali cell feed stream of aqueous alkali chloridewhich is substantially free of alkali hydroxide and hydrogen peroxide istaken from a lower region of the packed column.
 8. The method of claim7, wherein said contaminated stream is formed by the combination of aspent alkali chloride recycle stream from a COIL apparatus and adepleted anolyte stream from a chloralkali cell.
 9. The method of claim8, wherein said anolyte is combined with said recycle stream prior tothe introduction of the recycle stream to the top of the column.
 10. Themethod of claim 8, wherein said anolyte and said recycle stream arecombined by introducing both streams into the top of the column.
 11. Amethod of recycling an alkali chloride solution from the spent fuelstream of a Chemical Oxygen-Iodine Laser (COIL) apparatus comprisingseparating an aqueous alkali chloride stream having residual amounts ofalkali hydroxide and H₂O₂ from the spent fuel of a COIL apparatus,treating the alkali chloride stream by reacting the alkali hydroxide andH₂O₂ within said alkali chloride stream with Cl₂ and alkali hydroxide,both taken from a chloralkali cell, supplying the treated alkalichloride stream to the chloralkali cell as an anolyte feed solution,supplying a portion of the Cl₂ generated from the chloralkali cell tothe COIL apparatus, supplying a portion of the alkali hydroxidegenerated by the chloralkali cell to a peroxide generator in order togenerate basic hydrogen peroxide (BHP) solution, and supplying BHPsolution from the peroxide generator to the COIL apparatus as a fuelfeed stream.
 12. The method of claim 11, further comprising combining astream of depleted anolyte from the chloralkali cell with the alkalichloride stream.
 13. The method of claim 12, wherein the depletedanolyte stream and alkali chloride stream are combined prior totreatment of the alkali chloride stream.
 14. The method of claim 12,wherein the depleted anolyte stream and alkali chloride stream arecombined subsequent to treatment of the alkali chloride stream.
 15. Areactor for removing alkali hydroxide and hydrogen peroxide contaminantsfrom an aqueous alkali halide stream, comprising a reactor vessel; analkali hydroxide inlet in the upper region of the vessel; a contaminatedalkali halide inlet in the upper region of the vessel; a chlorine gasinlet in the lower region of the vessel; and a treated alkali halideoutlet in the lower region of the vessel.
 16. The reactor of claim 15,further comprising a depleted alkali chloride solution inlet in theupper region of the vessel.
 17. A basic hydrogen peroxide (BHP)recycling system comprising a chemical oxygen-iodine laser (COIL); aseparating apparatus which receives spent BHP from the COIL andseparates the spent BHP into a purified alkali hydroxide H₂O₂ stream,which is returned to the COIL, and an aqueous alkali chloride recyclestream having residual alkali hydroxide and H₂O₂; a chloralkali cell; areactor which receives the alkali chloride recycle stream from theseparating apparatus, a depleted anolyte stream from the chloralkalicell, a first alkali hydroxide stream from the chloralkali cell, and afirst Cl₂ gas stream from the chloralkali cell, and which evolves atreated alkali chloride stream substantially free of alkali hydroxideand H₂O₂ which is supplied to the chloralkali cell and an oxygen off gasstream; and a peroxide generator which receives a second alkalihydroxide stream from the chloralkali cell and produces a regeneratedstream of BHP, which is supplied to the COIL, wherein a second Cl₂ gasstream is supplied from the chloraikali cell to the COIL.
 18. The systemof claim 17, wherein the reactor is a packed column reactor havinginlets in the upper region of the column for receiving the alkalichloride recycle stream, the depleted anolyte stream, and the firstalkali hydroxide stream; an outlet in the upper region of the column foroffgassing of oxygen; an inlet in the lower region of the column forreceiving the first Cl₂ gas stream; and an outlet in the lower region ofthe column for the evolution of the treated alkali chloride stream.